This application claims the benefit of Korean Patent Application No. 2001-31878, filed in Korea on Jun. 8, 2001, and which is hereby incorporated by reference as it fully set forth herein.
1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon film, and more particularly, to a crystallization method using sequential lateral solidification (SLS).
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as active layer materials for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in switching devices of liquid crystal displays (LCDs). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCD.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having field effect mobility greater than 30 cm2/Vs together with low leakage current.
A polycrystalline silicon film is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grains boundaries the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interfaces between liquid and solid silicon such that the resulting grain boundaries are perpendicular to the interfaces. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude and a relative motion that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon re-crystallization.
FIG. 1A is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while FIG. 1B shows a plan view of a conventional mask 38 that is used in the apparatus of FIG. 1A. In FIG. 1A, the SLS apparatus 32 includes a laser generator 36, a mask 38, a condenser lens 40, and an objective lens 42. The laser generator 36 generates and emits a laser beam 34. The intensity of the laser beam 34 is adjusted by an attenuator (not shown) in the path of the laser beam 34. The laser beam 34 is then condensed by the condenser lens 40 and is then directed onto the mask 38.
Specifically referencing FIG. 1B, the mask 38 includes a plurality of slits xe2x80x9cAxe2x80x9d through which the laser beam 34 passes, and light absorptive areas xe2x80x9cBxe2x80x9d that absorb the laser beam 34. The width of each slit xe2x80x9cAxe2x80x9d effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between each slit xe2x80x9cAxe2x80x9d defines the size of the lateral grains growth of amorphous silicon crystallized by the SLS method. Referring not to FIG. 1A, the objective lens 42 is arranged below the mask and reduces the shape of the laser beam that passes through the mask 38.
Still referring to FIG. 1A, an X-Y stage 46 is arranged adjacent to the objective lens 42. The X-Y stage 46, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 so as to receive light from the objective lens 42. Although not shown in FIG. 1A, it should be understood that an amorphous silicon film is on the substrate 44, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser generator 36 and the mask 38 are typically fixed in a predetermined position while the X-Y stage 46 moves the amorphous silicon film on the sample substrate 44 in the x-axial and/or y-axial direction. Alternatively, the X-Y stage 46 may be fixed while the mask 38 moves to crystallize the amorphous silicon film on the sample substrate 44.
When performing SLS crystallization, a buffer layer is typically formed on the substrate 44. Then, an amorphous silicon film is deposited on the buffer layer. Thereafter, the amorphous silicon is crystallized as described above. The amorphous silicon film is usually deposited on the buffer layer using chemical vapor deposition (CVD). Unfortunately, that method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes de-hydrogenation, which results in a smoother surface on the crystalline silicon film. If the de-hydrogenation is not performed, the surface of the crystalline silicon film is rough and the electrical characteristics of the crystalline silicon film are degraded.
FIG. 2 is a plan view showing a substrate 44 having a partially crystallized amorphous silicon film 52. When performing SLS crystallization, it is difficult to crystallize all of the amorphous silicon film 52 at once because the laser beam 34 has a limited beam width, and because the mask 38 also has a limited size. Therefore, with large size substrate, the mask 38 is typically arranged numerous times over the substrate, while crystallization is repeated for the various mask arrangements. In FIG. 2, an area xe2x80x9cCxe2x80x9d that corresponds to one mask position is defined as a block. Crystallization of the amorphous silicon within a block xe2x80x9cCxe2x80x9d is achieved by irradiating the laser beam several times.
Crystallization of the amorphous silicon film will be explained as follows. FIGS. 3A to 3E are plan views showing one block of an amorphous silicon film being crystallized using a conventional SLS method. In the illustrated crystallization, it should be understood that the mask 38 (see FIGS. 1A and 1B) has three slits.
As is well known, the length of the lateral growth of a grain is determined by the energy density of the laser beam, by the temperature of substrate, and by the thickness of amorphous silicon film (as well as other factors). The maximum lateral grain growth should be understood as being dependent on optimized conditions. In the SLS method shown in FIGS. 3A to 3E, the width of the slits is less than or equal to twice the maximum lateral grain growth.
FIG. 3A shows an initial step of crystallizing the amorphous silicon film using a first laser beam irradiation. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 38 and irradiates one block of an amorphous silicon film 52 on the sample substrate 44. The laser beam 34 is divided into three line beams by the three slits xe2x80x9cA.xe2x80x9d The three line beams irradiate and melt regions xe2x80x9cDxe2x80x9d, xe2x80x9cExe2x80x9d and xe2x80x9cFxe2x80x9d of the amorphous silicon film 52. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film, i.e., complete melting regime.
Still referring to FIG. 3A, after complete melting, the liquid phase silicon begins to crystallize at interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains 58a and 58b proceeds from the un-melted regions to the fully-melted regions, so the grain boundaries are substantially perpendicular to the interfaces 56a and 56b (for clarity, only region xe2x80x9cDxe2x80x9d is shown with interfaces 56a and 56b and grains 58a and 58b, however, it should be understood that regions xe2x80x9cExe2x80x9d and xe2x80x9cFxe2x80x9d also have such features).
Lateral growth stops in accordance with the width of the melted silicon region when: (1) grains grown from interfaces collide near a middle section 50 of the melted silicon region; or (2) polycrystalline silicon particles are formed in the middle section 50 as the melted silicon region solidifies sufficiently to generate solidification nuclei. Further, the grown length of the laterally grown grains is usually 1.5 to 2 micrometers after the laser beam irradiation process. Namely, the grains laterally grown by the first laser beam irradiation can typically have the maximum length generally ranging from 1.5 to 2 micrometers (xcexcm).
Since the width of the slits xe2x80x9cAxe2x80x9d (see FIG. 1B) is less than or equal to twice the maximum lateral growth length of the grains, the width of the melted silicon region xe2x80x9cD,xe2x80x9d xe2x80x9cE,xe2x80x9d or xe2x80x9cFxe2x80x9d is also less than or equal to twice the maximum lateral growth length of the grains. Therefore, lateral grain growth stops when the crystalline silicon collide in the middle section 50, as shown in FIG. 3B.
FIG. 3B is an enlarged view of a portion xe2x80x9cC1xe2x80x9d of FIG. 3A. As mentioned, the first grains 58a grow from left to right, i.e., from the interface 56a to the center 50, and the second grains 58b grow from right to left, i.e., from the interface 56b to the center 50. When these grains 58a and 58b collide with each other at the center 50, the lateral grain growth stops, thereby defining first grain regions xe2x80x9cG1axe2x80x9d and xe2x80x9cG1b,xe2x80x9d and a first border xe2x80x9cH1.xe2x80x9d As discussed above, the grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 56a and 56b of the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam irradiation, crystallized regions xe2x80x9cD,xe2x80x9d xe2x80x9cE,xe2x80x9d and xe2x80x9cFxe2x80x9d each having the first grain regions xe2x80x9cG1axe2x80x9d and xe2x80x9cG1bxe2x80x9d and the first border xe2x80x9cH1xe2x80x9d are formed in one block. The number of these crystallized regions xe2x80x9cD,xe2x80x9d xe2x80x9cE,xe2x80x9d and xe2x80x9cFxe2x80x9d will vary depending on the number of the slits xe2x80x9cAxe2x80x9d of FIG. 1B.
FIG. 3C is an enlarged view of the portion xe2x80x9cC1xe2x80x9d and illustrates the mask adjustment for a next laser beam irradiation. In order to grow the grains, the mask 38 moves (relative to the amorphous silicon) to a position where the slit(s) xe2x80x9cAxe2x80x9d exposes a portion of the grain region xe2x80x9cG1a,xe2x80x9d the border xe2x80x9cH1,xe2x80x9d and the grain region xe2x80x9cG1b.xe2x80x9d Namely, when the width of the first grain regions xe2x80x9cG1axe2x80x9d and xe2x80x9cG1bxe2x80x9d is 2 micrometers (xcexcm), the slit xe2x80x9cAxe2x80x9d of the mask moves by 0.7 micrometers (xcexcm) in a transverse direction (i.e., in the x-axial direction). Then, a second laser beam irradiation is conducted to grow grains formed by the first laser beam irradiation, thereby resulting in grains 58c as shown in FIG. 3D.
FIG. 3D illustrates a partially crystallized amorphous silicon film after the second laser beam irradiation. During the second laser beam irradiation, the second laser beam irradiates portions of the first grain regions xe2x80x9cG1axe2x80x9d and xe2x80x9cG1bxe2x80x9d and previously unexposed amorphous silicon. The regions irradiated by the second laser beam are melted and crystallized, thereby defining second grain regions xe2x80x9cG2axe2x80x9d and xe2x80x9cG2b,xe2x80x9d and a second border xe2x80x9cH2xe2x80x9d thereof. During that time, the grains 58a generated by the first laser beam irradiation serve as seeds for the second crystallization. Thus, the grains 58c are formed by lateral grain growth of the grains 58a in the second melted regions. Namely, the silicon grains 58c formed by the second laser beam irradiation continue to grow along the silicon grains 58a formed by the first laser beam irradiation. Meanwhile, new silicon grains 58d are grown from a newly formed interface 56c. The lateral growth of the grains 58c and 58d stop when they collide with each other at a line 50b of the second border xe2x80x9cH2xe2x80x9d.
Accordingly, by repeating the steps of melting and crystallizing, one block of the amorphous silicon film is fully crystallized to form grains 58e as shown in FIG. 3E. Furthermore, the amorphous silicon film is converted into crystalline silicon film by block-by-block crystallization. Namely, the above-mentioned crystallization processes conducted within one block are repeated block by block across the amorphous silicon film. Therefore, the large amorphous silicon film is converted into a crystalline silicon film.
While generally successful, the conventional SLS method described above has disadvantages. Although the conventional SLS method produces relatively large size grains, the X-Y stage or the mask must repeatedly move a few micrometers to induce lateral grain growth. Therefore, the time required to move the X-Y stage or the mask 38 occupies a major part of the overall process time. This significantly decreases manufacturing efficiency.
To reduce the process time, a mask having closely spaced slits is often used. When a mask has a shorter distance between adjacent slits the mask can have more slits per unit area. Therefore, such a mask can reduce the number of laser beam shots and can increase productivity. However, a mask having closely spaced slits limits the grain length that can be achieved. This can reduce the suitability of the resulting crystalline silicon film for the active layer of driving devices, such as data driving circuits and gate driving circuits. However, crystalline silicon film formed by masks having closely spaced slits can still be useful for active layers of LCD panel switching devices (such as TFTs). This is because short grains (i.e., less than 1 micrometer) are still useful in such applications.
Accordingly, when forming TFT active layers, masks having closely spaced slits are usually used to reduce the overall processing time. However, when forming active layers for driving devices a mask having wider spaced slits are usually required. Thus, two masks, one for the TFTs and one for the driving circuits, are beneficial in some applications.
The masks for driving circuits have a relatively small number of slits per unit area. Furthermore, to make large grains the laser beam patterns of the second laser beam irradiation are required to overlap the center of the grain regions xe2x80x9cG1axe2x80x9d and xe2x80x9cG1bxe2x80x9d that were formed by the first laser beam irradiation (reference FIG. 3C). However, the masks for TFTs have a relatively large number of slits per unit area. Therefore, the process time of forming the TFT active layer is relatively small (the grain size is smaller than in the driving circuit).
In the conventional art, let us suppose that the mask moves a distance of 0.7 micrometers, i.e., the translation distance is 0.7 micrometers. Further, let us suppose that the length of the lateral grain growth is about 1.2 micrometers and that the slit width is 2 micrometers. Further assume that the distance between adjacent slits is 10 micrometers. Under these assumptions the laser beam will be shot about 16 times to crystallize one block. This process time is relatively long and the manufacturing efficiency is low.
Alternatively, if the distance between adjacent slits is 4 micrometers, the laser beam will be shot about 9 times to crystallize one block. Although the number of laser beam irradiation decreases, the resulting grains are smaller and the crystalline silicon film has more larger grain boundaries. These grain boundaries interrupt the carrier movement and decrease the carrier mobility. Therefore, such small grains are usually not adequate for driving circuits. Therefore, another mask for forming the crystalline silicon active layer adequate to the driving circuit is required. Using the different mask for the driving circuit, however, causes increasing the cost of production.
Therefore, a new method of crystallizing amorphous silicon using sequential lateral solidification (SLS) such that both TFTs and driving circuit active layer materials are formed at the same time and with improved manufacturing efficiency would be beneficial.
Accordingly, the present invention is directed to a method of crystallizing an amorphous silicon film using a sequential lateral solidification (SLS) that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a sequential lateral solidification (SLS) method which uses only one mask whatever for the TFTs and driving circuits such that it saves time in crystallizing an amorphous silicon film and obtain a productivity increase.
Another advantage of the present invention is to provide a method of crystallizing an amorphous silicon layer with increased manufacturing yield using the improved SLS method.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the method particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of crystallizing an amorphous silicon film using a mask that includes a light absorptive portion for blocking a laser beam and a plurality of Y-axially stripe-shaped light transmitting portions for passing the laser beam is provided. The method includes locating a substrate having an amorphous silicon film in a sequential lateral solidification (SLS) apparatus; dividing the amorphous silicon film into a driving region where driving devices are to be located and a display region where thin film transistors are to be located. Then, irradiating the driving region using a laser beam that passes through the mask such that laterally grown grains are formed that correspond to the Y-axially stripe-shaped light transmitting portion of the mask, wherein those laterally grown grains form a first grain region, a second grain region and a first middle section between the first and second grains regions. Then, X-axially moving the mask relative to the substrate by a translation distance that is less than half the width of the Y-axially stripe-shaped light transmitting portions and such that each of the Y-axially stripe-shaped light transmitting portion exposes a portion of the first grain region, the first middle section and the second grain region. Then, performing a second crystallization on the driving region such that the grains grow in second irradiated portions of the driving region, and irradiating the display region using a laser beam that passes through the mask such that the laterally grown grains are formed in the irradiated display region that corresponds to the Y-axially stripe-shaped light transmitting portion of the mask, wherein the laterally grown grains form a third grain region, a fourth grain region and a second middle section between the third and fourth grains regions. Then, X-axially moving the mask relative to the substrate by a translation distance that is more than half the width of the Y-axially stripe-shaped light transmitting portions and such that each of the Y-axially stripe-shaped light transmitting portion exposes a portion of the fourth grain region. Also, performing a second crystallization on the display region such that laterally growing grains grow in the second irradiated portions of the display region.
The method of crystallizing the amorphous silicon film further includes X-axially moving the mask a plurality of times to complete the X-axis directional crystallization, and then moving the mask in the Y-axial direction by the length of light transmitting portions after the amorphous silicon film is crystallized in the X-axial direction. Also, conducting a second X-axis directional crystallization after moving the mask in the Y-axial direction a translation distance that is less than half the width of the Y-axially stripe-shaped light transmitting portions (0.5 to 0.8 micrometers), while the translation distance is more than half the width of the Y-axially stripe-shaped light transmitting portions (1.6 to 1.8 micrometers). The length of lateral grain growth in the driving region of the amorphous silicon film is 10 to 13 micrometers, and the length of lateral grain growth in the display region of the amorphous silicon film is 1.5 to 3 micrometers. The width of the Y-axially stripe-shaped light transmitting portions of the mask is less than or equal to twice the maximum length of lateral grain growth that is to be grown by sequential lateral solidification (SLS). This width of the Y-axially stripe-shaped light transmitting portions of the mask is beneficially 2 micrometers. The distance between the Y-axially stripe-shaped light transmitting portions of the mask is about 10 micrometers. The length of the Y-axially stripe-shaped light transmitting portions of the mask is about 10 micrometers.
In another aspect of the present invention, a method of crystallizing an amorphous silicon film using a mask that includes a light absorptive portion for blocking a laser beam and a plurality of X-axially stripe-shaped light transmitting portions for passing the laser beam. The method includes locating a substrate having an amorphous silicon film in a sequential lateral solidification (SLS) apparatus; dividing the amorphous silicon film into a driving region where driving devices are located and a display region where thin film transistors are located, and then irradiating the driving region using a laser beam that passes through the mask such that laterally grown grains are formed in the irradiated driving region and that corresponds to the X-axially stripe-shaped light transmitting portion of the mask, wherein laterally grown grains form a first grain region, a second grain region and a first middle section between the first and second grains regions. Then, X-axially moving the mask a plurality of times by the length of X-axially stripe-shaped light transmitting portions to complete the X-axis directional crystallization in the driving region of the amorphous silicon film. Then, Y-axially moving the mask relative to the substrate by a translation distance that is less than half the width of the X-axially stripe-shaped light transmitting portions after the driving region is crystallized in the X-axial direction and such that each of the X-axially stripe-shaped light transmitting portion exposes a portion of the first grain region, the first middle section and the second grain region. Then, performing a second X-axial directional crystallization on the driving region such that laterally growing grains grow in irradiated portions of the driving region. Then, irradiating the display region using a laser beam that passes through the mask such that the laterally grown grains are formed in the irradiated display region that corresponds to the X-axially stripe-shaped light transmitting portion of the mask, wherein laterally grown grains form a third grain region, a fourth grain region and a second middle section between the third and fourth grains regions. Then, X-axially moving the mask a plurality of times by the length of X-axially stripe-shaped light transmitting portions to complete the X-axis directional crystallization in the display region of the amorphous silicon film. Then, Y-axially moving the mask relative to the substrate by a translation distance that is more than half the width of the X-axially stripe-shaped light transmitting portions and such that each of the X-axially stripe-shaped light transmitting portions expose a portion of the fourth grain region. Then, performing a second X-axis directional crystallization to the display region such that laterally growing grains grow in irradiated portions of the display region.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.